This invention concerns apparatus for limiting the amplitude of color signals in a color television camera or a color television signal processor. In particular, it relates to apparatus for detecting when the amplitude of one color signal has been limited in order to limit the amplitude of other color signals.
One step in the alignment of a television camera is to obtain proper color balance. This is performed by placing a white object in the field of view of the camera and either manually adjusting the gains of the various red, green and blue color signals or allowing the camera to automatically adjust the gains of the color signals to produce signal which will cause a white image of the object to be reproduced on a display device. The color signal levels produced by the color balance operation define maximum red, green and blue signal levels for the camera.
A problem may occur, however, when, after color balance calibration, an image viewed by the camera contains objects which are brighter than the object that was used for the color balance operation or which have higher color saturation values than are allowed by the respective red, green and blue signal levels developed from the color balance operation. When objects in the image are brighter than the object used for the white balance calibration, the respective red, green and blue signals which describe these objects may be clipped. That is to say, held at the maximum digital or analog value that may be provided by the camera.
When, due to a bright object in the camera's field of view, one of the color signals is clipped while the other color signals are not clipped, errors in hue can occur in the resulting image. These errors tend to desaturate the image of the bright object causing, for example, a solid yellow object to appear mottled with white. In addition, if significant texture information was contained in the signals that were clipped, this may be lost in the reproduced image of the object.
Problems of the type described above also occur in television receivers. It is commonly known, for example, that supersaturated color images may be produced by controlling the amplitude of the color-burst component relative to image-signal components of the television signal. If the maximum amplitude of the digitized color signals are set relative to the burst component, then the amplitudes of supersaturated portions of the image may be clipped, producing the same desaturation and loss of image detail artifacts as described above.
A system which detects the clipping of color signals is described in an article by C. L. Novac et al. entitled, "Obtaining Accurate Color Images For Machine Vision Research," Proceedings of the International Society for Optical Engineering, Vol. 1250, pp. 54-68, which is hereby incorporated by reference for its teachings on color signal processing. This paper describes an algorithm for color machine vision which attempts to classify objects in an image by their color.
Color signal clipping is described in this reference as a type of distortion which can be detected but not corrected. Two solutions to this problem are proposed. First, to eliminate the effective pixels from further consideration and, second, to make educated guesses as to what the true hue of the distorted pixels may be. The paper gives no guidance as to how these educated guesses might be made.
Another system which attempts to limit the effects of bright objects in an image is described in U.S. Pat. No. 5,083,216 entitled, AUTOMATIC BRIGHTNESS ALGORITHM IN A SLIDE-TO-VIDEO TRANSFER UNIT. This system repeatedly scans an image through a video camera varying the aperture of the camera until the percentage of peak values which are above a defined maximum value is less than or equal to three percent of the sample values in the image.